The present invention relates to a power converter for energizing an AC load (e.g., motor), particularly relating to a voltage-source power converter which ensures a sufficient start torque for an induction motor.
A frequency-variable and output-variable power converter is advantageous for energizing or actuating an AC motor. According to such a power converter, (1) it is possible to effectively suppress an overcurrent at the time of starting of the motor; (2) it is possible to vary the rotating speed of the motor by changing the frequency of a converter output, thereby saving energy loss and achieving a variable speed operation; and (3) a high-grade controllability as that attained in a DC motor control system may be obtained when a suitable control scheme is applied. A voltage-source inverter, current-source inverter, load-commutated inverter or cycloconverter is generally used as the frequency-variable and output-variable power converter.
A current-source inverter, load-commutated inverter or cycloconverter is a sort of a current controlled converter. Although the use of such a current controlled converter provides an AC motor control system with various advantages, it invites a certain problem. That is, when the operating frequency of an AC motor is high, since the influence of characteristics of the motor becomes prominent, difficulties are involved in determining the circuit constant of a power converter main circuit or in assuring a sufficient control tolerance.
On the other hand, a voltage-source inverter is a sort of a voltage controlled converter. A voltage controlled converter may ideally serve as an AC power source for actuating an AC motor.
FIG. 1 shows a conventional voltage-source inverter for actuating an AC motor. In FIG. 1, a DC power is obtained from an AC power line 11 through a rectifier 12. The DC power is supplied to an inverter 15 through a filter circuit of a reactor 13 and capacitor 14. Inverter 15 generates from the DC power an AC current Iac having a given frequency which is variable. Current Iac is supplied to an induction motor (AC load) 16 which may have a power capacity of thousands kW order.
An AC voltage Vac applied to motor 16 is detected as a voltage signal E23 via a potential transformer 23. Signal E23 is rectified by a rectifier 24. Rectifier 24 delivers a feedback signal Efbk whose DC potential corresponds to the amplitude of AC voltage Vac. Signal Efbk is supplied to the negative input of a comparator 17. The positive input of comparator 17 receives a voltage reference signal Eref. Signal Eref is obtained via an input limiter 22 from a reference value designator 21. The DC potential of an output E21 from designator 21 is optionally changed by the manipulation of an operator of the power converter. The maximum and minimum DC potentials of output E21 are restricted to predetermined values by the circuit operation of limiter 22, and a potential limited output E22 from limiter 22 is supplied as the signal Eref to comparator 17. Comparator 17 generates a potential difference (Eref-Efbk) between the inputted signals Eref and Efbk. This potential difference (Eref-Efbk) is amplified through a voltage control circuit 25 and converted into a current reference signal Iref.
Signal Iref is inputted to the positive input of a comparator 18. The negative input of comparator 18 receives a current feedback signal Ifbk. Signal Ifbk is obtained from a current transformer 26 which is located at the DC current path of rectifier 12. Thus, the potential of signal Ifbk represents the magnitude of a DC current Idc of rectifier 12. Comparator 18 generates a potential difference (Iref-Ifbk) between the inputted signals Iref and Ifbk. This potential difference (Iref-Ifbk) is amplified through a current control circuit 27 and converted into a phase control signal E27. Signal E27 is supplied to one input of a phase control circuit 29. The other input of circuit 29 receives a voltage signal E28. Signal E28 is obtained from a potential transformer 28 which is located at the circuit of AC power line 11. Signals E27 and E28 are converted by phase control circuit 29 into gate pulses EX which are used for triggering thyristors in the rectifier 12.
Output E22 from limiter 22 also serves as a frequency reference signal Fref. Signal Fref is supplied to a signal oscillator 31. Oscillator 31 generates an AC signal S0 having a frequency corresponding to the operating frequency of motor 16. Signal S0 is converted by a gate pulse generator 32 into gate pulses EY which are used for triggering GTOs (gate turn-off thyristors) in the inverter 15.
FIG. 2 shows a typical circuit configuration of the main portion of a 3-phase voltage-source inverter. In FIG. 2, rectifier 12 is formed with thyristors 121 to 126. Rectifier 12 serves as a DC power source which can control the DC output current Idc. Such a DC power source may be a chopper circuit provided with a battery or the like. Inverter 15 employs GTOs 151 to 156 as main switching elements. Diodes 251 to 256 are cross-coupled with GTOs 151 to 156, respectively. Here, self extinguishing type semiconductor elements such as GTOs, GTRs or the like, or a combination of thyristors and their associated forced commutation circuit, may be used for the main switching elements. In any case, these elements can be made conductive or nonconductive by gate pulses and, therefore, they are called "controllable rectifying elements." The following description will be given to a case wherein GTOs are used as the main switching elements.
Although a voltage-source inverter is suitable to a power source of an AC motor, it lacks a sufficient current control function. Thus, the voltage-source inverter involves the following problems when it is applied to an AC motor control apparatus.
(1) Before and immediately after the start of rotation, since no or little counter electromotive force (induction voltage) is generated from the AC motor, the output circuit of the inverter is substantially short-circuited with a low impedance of the motor. This fact disturbs an accurate control of voltage and frequency references based on the conditions of the AC motor or on the conditions of machines to be actuated by the AC motor. From this, it is hard to achieve starting of the motor with a rated current value.
(2) Unless the control for starting the motor with a rated current as mentioned above is effected, a sufficient starting torque of the motor cannot be obtained by a voltage-source inverter having a normal power capacity. Or, an excessive power capacity is required to the inverter for ensuring a sufficient starting torque of the motor. Accordingly, so long as the above motor starting control with a rated current cannot be achieved, an AC motor control apparatus for ensuring a sufficient starting torque becomes very costly.
(3) Conventionally, a PWM (pulse width modulation) control enables to achieve the starting of an AC motor with a certain torque, while avoiding an excessive inverter output current. However, such a PWM control inherently requires the proper use of small-capacity main switching elements, such as GTRs or power MOSFETs, which have to be operated in chopping with a high frequency. From this, it is difficult to apply a PWM control method to a voltage-source inverter using GTOs or thyristors. Further, even if GTRs or power MOSFETs are used for the voltage-source inverter, an output current from the inverter involves large ripples, resulting in reducing actually available output power of the inverter.